A video sequence consists of a series of still pictures or frames. Video compression methods are based on reducing the redundant and perceptually irrelevant parts of video sequences. The redundancy in video sequences can be categorized into spectral, spatial and temporal redundancy. Spectral redundancy refers to the similarity between the different color components of the same picture. Spatial redundancy results from the similarity between neighboring pixels in a picture. Temporal redundancy exists because objects appearing in a previous image frame are also likely to appear in the current image frame. Compression can be achieved by taking advantage of this temporal redundancy and predicting the current picture from another picture, called a reference picture. Further compression is achieved by generating motion compensation data that describes the motion between the current picture and the reference picture.
Video compression methods typically differentiate between pictures that utilize temporal redundancy reduction and those that do not. Compressed pictures that do not utilize temporal redundancy reduction methods are usually called INTRA- (or I-) frames or pictures. Temporally predicted images are usually forwardly predicted from a picture occurring before the current picture and are called INTER- or P-frames. A compressed video clip typically consists of a sequence of pictures, which can be roughly categorized into temporally independent INTRA pictures and temporally differentially coded INTER pictures. INTRA pictures are typically used to stop temporal propagation of transmission errors in a reconstructed video signal and to provide random access points into the video bit-stream. Since the compression efficiency provided by INTRA pictures is normally lower than that provided by INTER pictures, they are generally used sparingly, especially in low bit-rate applications.
A video sequence can be composed of many camera scenes or shots. A shot is defined as a set of continuous frames or pictures taken with one camera. Generally, the frames within one shot are highly correlated. However, in a typical video sequence, the picture contents are significantly different from one scene to another and therefore the first picture of a scene is typically INTRA-coded. Changes between the different shots in a video sequence are referred to as “scene transitions”. Scene transitions may take a number of different forms. For example, one shot may end and another may begin abruptly at a “scene cut”. In other cases, the scene transition is gradual and occurs over more than one frame. Examples of gradual scene transitions are “dissolves”, “fades” (fade-in, fade-out) and “wipes”.
Compressed video is easily corrupted by transmission errors, mainly for two reasons. Firstly, due to the utilization of temporal predictive differential coding (INTER frames), errors are propagated both spatially and temporally. Secondly, the use of variable length codes increases the susceptibility of the video bit-stream to errors. There are many ways for a receiver (video decoder) to address the corruption introduced in the transmission path. In general, on receipt of a signal, transmission errors are first detected and then corrected or concealed by the decoder. The term “error correction” refers to the process of recovering the erroneous data perfectly, as if no errors had been introduced in the first place, while “error concealment” refers to the process of concealing the effects of transmission errors so that they are hardly visible in the reconstructed video sequence.
Currently, the video decoder being developed by the Joint Video Team (JVT) of the ISO/IEC Motion Pictures Expert Group and the ITU-T Video Coding Experts Group for the ITU-T H.264/MPEG-4 part 10 AVC video codec lacks a method for deciding how transmission errors in INTRA-coded frames and scene transition frames are to be concealed and it is in this context that the current invention has been developed.